You need a way to get audio out of the IP Office and onto the network to be broadcast to the speakers.

So just like traditional paging, need a Paging interface that can take audio from a loop start trunk port, an analogue station port programmed as a paging port, an analog station port that rings, or an IP device that registers as an IP extension.

Once you have your audio, what is the speaker system that you are using? Bogen? Valcom? Viking? Algo? All of these system are going to have a Gateway that receives your audio and broadcasts it to the speakers. If you are using an IP extension, the gateway might be the same device as your paging interface.